In the typical papermaking machine, an aqueous suspension of fibers is transformed into a paper web as it is processed through the different sections of the machine. One section of the papermaking machine is the dryer section wherein a wet paper web is passed about and held in intimate heat transfer relation with upper and lower arrays of heated cylinders in order to remove water from the paper web. The dryer section normally includes an upper and lower array of heated cylinders arranged and spaced in staggered, parallel rows which have a solid imperforate surface for contacting the paper web. Several dryer sections may be found in a papermaking machine in series and stages. The paper web is passed generally to and fro between the arrays of dryer cylinders in a generally serpentine manner to ensure that both sides of the paper web contact the cylinders. As the paper web passes over the dryer cylinders, it is held in intimate heat transfer contact therewith by a belt, commonly referred to as a dryer felt or dryer fabric which has been made endless by techniques which are well known in the field of papermaker's felts and clothing.
In the past, dryer fabrics generally have been substantially impervious structures of either woven or needle construction. However, the impervious structures, commonly known as dryer felts, do not ventilate sufficiently and thus serve to confine heated vapors in certain "pockets" created in the dryer section which cause uneven drying and affect paper quality. Thus, the trend is toward open weave fabrics which have been found to have desirable characteristics and many non-woven structures, such as needle felts, plastic perforated and helical belts, and the like have also been found suitable due to their increased permeability. Typically, these plastic, non-woven fabrics have yielded permeabilities as high as a thousand cublic feet per minute.
The non-woven plastic helical fabric is desirable because it has fewer if any "knuckles," as in the case of woven fabric, and thus provides increased surface area for contacting and holding the paper. This paper supporting surface is also smoother and reduces markings on the paper. The plastic material and belt construction hold up extremely well under the stresses encountered when traveling endlessly at high speeds, typically 3000 fpm, about the belt rollers in contacting the paper web.
The high permeability of non-woven plastic belting provides increased pocket ventilation and hence drying, but can also lead to increased fluttering of the paper web sheet against the dryer fabric through a phenomena known as air "pumping." This is due to frictional drag on the air surrounding the moving fabric which causes the air to move with the fabric. As the fabric contacts the cylinders or belt rolls, this air is forced through the fabric with the air movement away from the roll or dryer at a converging nip and toward the roll or dryer at a diverging nip. The effect of these forces is the net inflow of air into a dryer pocket, resulting in an outlow of air at the front and back sides of the machine. This turnover of air or air "pumping" is appreciable with highly permeable fabrics travelling at high speeds which can cause disruptive sheet flutter. If a highly permeable dryer fabric is operated at high speeds with a paper web of low strength, the paper web may break, be damaged, or be marked by fluttering against the fabric.
Heretofore, there has been no practical manner of varying the permeability of the basic fabric and, thus, provide fabric for a complete range of applications and control sheet flutter. For example, a dryer section closest to the wet section of a papermaking machine typically requires a fabric characterized by low permeability owing to its relative weak strength of the paper at this stage. High permeability is desired at dryer sections at the latter drying stages and for heavier paper grades. Finer paper grades do not contain as much water and thus do not require a fabric of high permeability nor is such desired since paper flutter of the light sheet is likely.
In an unrelated use, the wire mesh openings of a wire conveyor belt have been obstructed by plastic rods and the pressure of freezing air flow therethrough increased to minimize the variability of the air flow and provide the the result of uniform freezing of layered food conveyed thereon in U.S. Pat. No. 4,186,566, which is directed to apparatus for quick freezing foods.
Accordingly, an important object of the present invention is to provide a construction for a plastic non-woven dryer fabric for a papermaking machine and method by which a desired permeability characteristic may be built into the fabric.
Another important object of the present invention is to provide a non-woven dryer fabric and method having a permeability ranging from as low as 50 to as high as 1000 cfm.
Yet another important object of the present invention is to provide a plastic non-woven dryer fabric with a closed mesh which advantageously withstands the high stresses and temperatures on a papermaking machine.
Still another important object of the present invention is to provide a non-woven polymeric dryer fabric having a closed mesh to control sheet flutter and provide a smooth surface to dry and produce very fine paper.
Yet another important object of the present invention is the method of constructing a basic dryer fabric wherein the permeability of the fabric may be subsequently altered to provide a fabric having a desired permeability for a particular application.